Short loop ADSL power spectral density management

Abstract

A digital subscriber line (DSL) modem for a service area interface, which controls the power of its downstream transmissions to minimize far-end crosstalk (FEXT), is disclosed. The disclosed modem has an interface to a low-attenuation upstream facility, such as fiber optic, and includes a digital transceiver and an analog front end that is coupled to a twisted-pair wire facility in a binder. The modem also includes a memory location for storing the feeder distance between a DSL central office and the service area interface, the service area interface also coupled to a subscriber of the CO-fed communications via twisted-pair wire. Power cutback levels are applied to the downstream transmissions from the modem according to the feeder distance, so that the FEXT on the CO-fed signal is minimized without undue data rate degradation.

1 . A method of establishing a digital subscriber line communications link with a subscriber, from a location disposed at a feeder distance from a first central office, comprising: 
 initializing communications with the subscriber over a first communications facility;    determining a power cutback level for transmissions to the subscriber based on the feeder distance; and    transmitting payload data received from a second central office to the subscriber over the first communications facility according to the power cutback level.    
   
   
       2 . The method of  claim 1 , wherein the payload data is received over a fiber optic communications facility; 
 and wherein the first communications facility comprises twisted-pair wire.    
   
   
       3 . The method of  claim 1 , wherein the initializing, determining, and transmitting steps are performed by a digital subscriber line modem at a service area interface; 
 wherein the first communications facility comprises twisted-pair wire.    and wherein communications from the first central office are received by the service area interface over a feeder facility comprising twisted-pair wire, and are forwarded from the service area interface to a second subscriber from the service area interface over a second communications facility comprising twisted-pair wire.    
   
   
       4 . The method of  claim 1 , wherein the initializing, determining, and transmitting steps are performed by a digital subscriber line modem at a service area interface; 
 and further comprising: 
 storing, in memory at the service area interface, a parameter value indicating the feeder distance; and  
 before the determining step, retrieving the stored parameter value indicating the feeder distance.  
   
   
   
       5 . The method of  claim 4 , further comprising: 
 during the initializing step, measuring the power of signals received from the subscriber over the first communications facility;    and wherein the determining step determines the power cutback level based on the feeder distance and based on the measured power.    
   
   
       6 . The method of  claim 1 , wherein the determining step is performed during the initializing step.  
   
   
       7 . The method of  claim 1 , wherein the transmitting step transmits payload data over a plurality of subchannels; 
 and wherein the determining step determines power cutback levels that vary over the plurality of subchannels.    
   
   
       8 . A digital subscriber line modem for a service area interface located at a feeder distance from a first central office, comprising: 
 an upstream interface, for coupling to a data source;    an analog front end, for coupling to a subscriber via a first distribution communications facility;    a digital transceiver, coupled to the upstream interface and to the analog front end, for digitally processing data corresponding to signals received at the upstream interface and to be transmitted to the subscriber, the digital transceiver comprising: 
 circuitry for associating digital data corresponding to the signals to be transmitted with a plurality of subchannels;  
 gain scaling circuitry for applying gain to the signals to be transmitted over the plurality of subchannels according to a power cutback level that corresponds to the feeder distance; and  
 modulation circuitry for modulating the plurality of subchannels according to the digital data to be transmitted.  
   
   
   
       9 . The modem of  claim 8 , wherein the transceiver further comprises: 
 power cutback control circuitry, for selecting the power cutback level to be applied to the gain scaling circuitry.    
   
   
       10 . The modem of  claim 9 , wherein the transceiver further comprises: 
 a power cutback level look-up table, for storing a plurality of power cutback levels to be applied to the gain scaling circuitry;    wherein the power cutback control circuitry selects from among the plurality of power cutback levels responsive to the feeder distance.    
   
   
       11 . The modem of  claim 10 , wherein the transceiver further comprises: 
 a memory for storing the feeder distance.    
   
   
       12 . The modem of  claim 10 , wherein the transceiver further comprises: 
 a memory for storing the feeder distance.    
   
   
       13 . The modem of  claim 9 , wherein the transceiver further comprises: 
 a power cutback level look-up table, for storing a plurality of power cutback levels to be applied to the gain scaling circuitry, each of the plurality of power cutback levels including varying power cutback values over the plurality of subchannels.    wherein the power cutback control circuitry selects from among the plurality of power cutback levels responsive to the feeder distance.    
   
   
       14 . The modem of  claim 8 , wherein the transceiver comprises: 
 programmable logic circuitry; and    program memory for storing instructions executable by the programmable logic circuitry, so that the programmable logic circuitry operates as the encoding circuitry, the gain scaling circuitry, and the modulation circuitry.    
   
   
       15 . The modem of  claim 8 , wherein the analog front end comprises: 
 a line driver and transceiver circuit, having an output and an input coupled to the first distribution communications facility.    
   
   
       16 . The modem of  claim 15 , wherein the analog front end comprises a hybrid circuit for coupling the line driver and transceiver circuit to the first distribution communications facility.  
   
   
       17 . The modem of  claim 8 , wherein the transceiver further comprises: 
 circuitry for digitally processing signals received from the subscriber over the first distribution communications facility, the digital processing comprising measuring the power of the received signals during initialization of a communications session.    
   
   
       18 . The modem of  claim 17 , wherein the transceiver further comprises: 
 power cutback control circuitry, coupled to the circuitry for digitally processing received signals, for selecting the power cutback level to be applied to the gain scaling circuitry responsive to the feeder distance, and also responsive to the measured power of the received signals during initialization.    
   
   
       19 . The modem of  claim 8 , wherein the upstream interface is for coupling to a data source via a fiber optic facility; 
 and wherein the first distribution facility comprises twisted-pair wire within a binder.    
   
   
       20 . The modem of  claim 19 , wherein the first central office is coupled to a second subscriber via a first feeder facility, comprising twisted-pair wire, disposed between the first central office and the service area interface, and via a second distribution facility, comprising twisted-pair wire, disposed in the binder with the first distribution facility.